1. Field of the Invention
The present invention relates to a drive controller of a lens apparatus performing manual focus (MF) and auto-focus (AF) by moving different lens units from each other.
2. Description of the Related Art
Many lens systems of a rear focus type, in which a focus lens unit is arranged closer to an image than a variable lens unit, are incorporated in zoom lenses of an auto-focus type since such a lens system offers an advantage of reducing the size and weight of the focus lens unit and quickly achieving focusing.
Whereas, many lens systems of a front cell focus type, in which the focus lens unit is arranged closer to an object than the variable lens unit, are incorporated in zoom lenses of a manual focus type since such a lens system permits a front cell lens unit to move the same quantity with respect to subjects lying at the same object distance regardless of magnification-varying states, and is accordingly advantageous to manual focus.
Japanese Patent Publication No. 2561637 discloses a zoom lens having a structure where a lens unit closer to an image than the variable lens unit is used as an auto-focus lens unit and another lens unit closer to an object than the variable lens unit is used as a manual focus lens unit, and where one of these two focusing methods can be selected where appropriate, depending on focusing purposes.
U.S. Pat. No. 4,609,260 discloses a control method in which the auto-focus lens unit is forcefully returned to its reference position (a position serving as a reference for performing a positional control of the lens unit) upon switching from an AF mode to an MF mode in the zoom lens disclosed in the above-mentioned patent document.
Unfortunately, when the auto-focus lens unit is simply moved to its reference position in a forceful manner, as in the zoom lens disclosed in U.S. Pat. No. 4,609,260, upon switching from the AF mode to the MF mode, a focus state of the lens apparatus achieved by auto focus is changed to a non-focus state of the same, thereby resulting in un-focused pictures until focusing is manually achieved in the MF mode.